This invention relates to magnetic tagging.
More particularly, in different but related aspects thereof, this invention relates to articles comprising magnetic tags, to magnetically tagged articles, to systems comprising such articles and a reading and/or writing assembly therefor, to mating or interlocking first and second parts with means preventing inadvertent re-use of second parts that had been previously used, to methods for encoding a plurality of articles, to methods for correct registration of a first part with a second part, and to methods preventing re-use of a disposable part adapted for mating or interlocking with a second part.
Various non-contact identification systems are currently available. The most extensively used is the optical bar code which has the advantage that the tags can be produced in large quantities at low cost by a printing process. It is easy to copy the tag and once printed the code cannot be altered. As the system is optical there must be line-of-sight between the reader and the tag. Reading, which usually relies on scanning laser beams, is disrupted by dirt or fluid on the tag. The cost and size of the readers makes bar code technology unsuitable for any system seeking to prevent re-use or to registration systems.
Radio Frequency Identification Tags employ electromagnetic radiation to couple the reader to the tag, and so do not rely upon line of sight. The tags use integrated electronic circuits and in some systems, such as those supplied under the “Tag-it” Trademark by Texas Instruments, allow writing as well as reading. A significant drawback to such devices is that they do not tolerate gamma sterilisation, and so this technology would not be suitable for connectors used to supply food and medicinal substances, which often require such sterilisation.
A number of systems employing magnetic materials have also been proposed.
Thus, WO 93/14474 (RSO Corporation NV) describes a system that generates an electromagnetic field, to which individual transponder units including means storing identification data respond to transmit a signal, which is then received by a receiver. The transponders also carry a magnetic element, the properties of which are influenced by an external magnetic field in an interrogation zone, to modify the transponder signal.
U.S. Pat. No. 4,075,618 (Montean et al.) describes an antipilferage magnetic tag detected in an interrogation zone producing an alternating magnetic field by high-order harmonics produced by the presence of the tag. The tag has an asymmetrically shaped piece of low-coercive force ferromagnetic material, including a centre section and flux concentrator sections at opposite ends of this section.
U.S. Pat. No. 6,371,379 (Dames et al.) discloses a magnetic tag having a first region of soft magnetic material with high-permeability, low-coercivity and a non-linear B-H characteristic, and a second adjacent region of permanently magnetisable material having at least three discrete regions of magnetic bias material, in which the discrete regions exhibit different levels and direction of bias. When the tag is interrogated by a constant frequency alternating magnetic field, the magnetic bias levels of these discrete regions are overcome at different times in the interrogation cycle. Because the hard magnetic material saturates the soft magnetic material, it does not generate a harmonic signal until the external alternating field is equal and opposite to the bias of a particular such discrete region. Thus the different bias levels of the discrete regions of hard magnetic material making up the tag enable the tag to be encoded. As a practical matter, such tags are generally limited to between 5 and 10 codes. Because the same alternating field is applied to all the discrete regions at the same time, the individual discrete regions cannot be written to or re-programmed in situ other than by equating all the bias levels to an identical value.
U.S. Pat. No. 6,144,300 (Dames) discloses what in effect is a magnetic analogue of an optical bar code reader system. Soft magnetic elements are arranged in a linear array and read by mechanically scanning a coil along the array, the coil generating a magnetic null enabling the elements of the array to respond to the applied field. The spatial arrangement of the elements in the linear array defines a code similar to a bar code.